The goal of these studies is to increase understanding of the pathogenesis, and to improve diagnostic, therapeutic and preventative measures for parasitic diseases. Continued improvements have been made in diagnostic assays for cryptosporidiosis (based on 23 kD protein antigen), giardiasis (an ELISA to detect antigen in the stool) strongyloidiasis (immediate hypersensitivity skin test response to larval metabolic antigen), schistosome hepatic fibrosis (ultrasonography) and filariasis (200 kD circulating antigen with phosphocholine determinants). Practical applications of all these assays are underway. Therapeutic trials in progress include the first use of ivermectin in bancroftian filariasis and strongyloidiasis, use of praziquantel in cerebral cysticercosis, study of prophylactic DEC to prevent loiasis in Peace Corps volunteers in Africa, and combination therapy with heat and long-term intravenous pentostam in patients with Diffuse Cutaneous Leishmaniasis. The first steps towards evaluating a malaria vaccine for humans have been taken with Phase I safety and immunogenicity studies of a recombinant sporozoite vaccine in normal volunteers. Pathogenesis studies have focused both on differential host immune responsiveness to parasite antigens (leishmaniasis, lymphatic filariasis, loiasis, onchocerciasis) and on possible influences of parasite strain variation defined by differences in DNA, antigenic and behavioral makeup (giardia, leishmania). Use of human volunteers infected with different strains of giardia demonstrated clearly the different pathogenetic potential of different parasite strains.